Erbium fiber lasers emitting in the 1.55 .mu.m wavelength region have been under intensive research for applications in telecommunications, spectroscopy and fiber sensors. To achieve single-frequency operation, fiber lasers constructed with either Fabry-Perot (FP) cavities or ring cavities have wavelength filtering components such as external gratings (L. Reekie, R. J. Mears, S. B. Poole, and D. N. Payne (1986) J. Lightwave Tech., LT-4, p.956), in-fiber gratings (J. T. Kringlebotn, P. R. Morkel, L. Reekie, J. L. Archambault, and D. N. Payne (1993) 19th European Conference on Optical Communication, 2:65; G. A. Ball and W. H. Glen (1992) J. Lightwave Tech. 10(10): 1338-43; G. A. Ball and W. W. Morey (1992) Conference on Optical Fiber Communications (OFC'92) session WA3, p.97), and Fabry-Perot filters (FFPs) (G. Grasso, A. Righetti, and F. Fontana (1991) 17th European Conference on Optical Communication, 1:149. and C. V. Poulsen and M. Sekja (1993) IEEE Photonics Tech. Lett. 5:646). Wavelength is tuned by filter adjustments. Recently, (P. Laporta, S. Taccheo, S. Longhi, and O. Svelto (1993) Optics Lett. 18:1232 reported single longitudinal mode operation of a microchip erbium:ytterbium (Er:Yb) phosphate glass laser in a 200-.mu.m FP cavity without additional filtering. The ultra short length of the active medium in this bulk glass laser allows single-frequency, narrow-bandwidth (reportedly 0.85 kHz) operation tunable over about 1 nm by changing the temperature of the microchip. Prior to the present invention, lasing had not been demonstrated in fiber lasers with fiber lengths less than about a few centimeters even when high gain Er:Yb codoped fiber was employed. Kringlebotn et al., supra.